Rechargeable batteries are used in many applications. One such application is in hybrid vehicles. In these vehicles, a plurality of individual battery cells are connected in series to provide a battery stack having a desired output voltage. A large number of cells may be connected in series such that the total potential difference developed across the battery stack is on the order of 150-600 volts, for example.
In general, it is desirable to monitor the voltage, the impedance and the state of charge of each individual cell in the battery stack. An excessive voltage may indicate thermal runaway, in which the cell has the potential to catch on fire or to explode. A low voltage may indicate a discharged condition. A typical requirement is to measure all cell voltages in the battery stack and the battery stack current more or less simultaneously, i.e., within a few microseconds, and to repeat the measurement on the order of every 100 milliseconds. This permits the state of charge and the impedance of the cells in the battery stack to be determined accurately.
Techniques for monitoring low voltage batteries are known in the art. However, significant problems are presented in monitoring a battery stack of several hundred volts. It is desirable to implement a battery monitoring system with low voltage circuitry, such as circuitry that operates at or below 16-30 volts. In addition, it is desirable to avoid the need for multiple isolation devices such as optical isolators. Accordingly, there is a need for novel battery monitoring apparatus and methods.